Crimping
Crimping
The first generation of vascular grafts were made of woven fabrics that were hand-sewn with the edges to prevent fraying. Major difficulties arose in the application of these early grafts as a result to their lack of compression resistance and their tendency. In 1955 james – topp introduced a method of crimping . Their discovery accrued when braid- nylon tubes placed over glass rod were being soaked in formic acid to weld the edges and thereby prevent fraying, and removal of the tube from the glass rod. So crimp is removed at implantation by the surgeon but the elasticity of the grafts depends on the elastic modulus of the material The formation of an accordion pleat which were made permanent by heat-setting at 30ºc for 30 min. (98) crimping is affected by the application of internal steam pressure in a mould designed according to the required configuration . The process normally produces a circular crimp, The technique is especially suited to velour grafts since the velour feature is well preserved . The thread winding process by contracting, produces only a helical – crimp configuration and tends to flatten the filamentous feature of the velour. In the thread – winding process crimping is done by spirally winding a thread around the graft ,the graft is being compressed at both ends and heat –set (101) Heat- sensitive thermoplastic fibers are generally given a heat – setting finish to ensure a stablse fabric or as a means of producing special shape. A major reason for heat- setting is to introduce dimensional stability by using high temperatures besides that heat – setting contributes to wrinkle resistance, elastic recovery which aids in size retention and relatively permanent design details such as pleat (plisse). Planned creases or surface embossing fabrics can be heat – set in a pleats shape and pressed in , they can even be made to assume an end – use shape (98) where please effects are a chivied in fabrics by heat setting at 220-230ºc (or by imprinting them with chemicals that cause the fabric to shrink when these chemicals are printed in design , some areas of the fabrics shrink while others do not , this cause untreated are as to pucker or puff up between the treated areas (3) Crimping improves the grafts bending ability without kinking , besides reducing the likelihood of kinking crimping has added advantage of improving the longitudinal compliance , and vascular handle
